Watermarking involves embedding information in data so that the information is hidden in the normal use of that data. Ideally, the information is embedded so that 1) it does not significantly reduce data quality, 2) the information can be recovered and 3) it is robust both to malicious attacks and nonmalicious attacks such as typical processing operations that may be performed on the data. Thus, watermarking involves a trade-off among data quality, robustness and capacity of information carried by the watermark.
There are several challenges in watermarking a motion picture. A motion picture is any sequence of images that, when played back at a designated frame rate, gives the appearance of motion. Such images may originate from a camera and may be recorded on film, or analog video tape, or digital video tape, or in a data file on a computer readable medium, or may originate in a computer, such as with computer-generated animation or special effects.
The embedded information of a watermark appears as noise added to each image. If this noise is the same in each image in a motion picture, then it may become visually perceptible during playback of the motion picture as images change but the noise does not. In particular, the objects in the images may appear to be moving behind the noise. If the watermarks in different images are the same, or in some cases merely temporally correlated, then the watermarks may become visually perceptible.
Because each image in a motion picture may be valuable, especially during production and editing of the motion picture prior to its formal release, each image of the motion picture should be watermarked. However, watermarking each image differently can be computationally intensive and adds both to the complexity and storage requirements of any process for detecting watermarks.
Motion pictures also are susceptible to time-domain image processing and other operations, in addition to signal domain and frequency domain operations on individual images. Such operations may include, but are not limited to, changes in the color space, bit depth, pixel resolution or temporal resolution (e.g., using frame rate conversion, whether by interpolation or by introduction of repeated fields or frames, or by removal of fields or frames), or changes due to rotation, scaling, cropping, morphing, warping and translation. Such operations also may include compression, especially forms of compression that reduce interframe redundancies in addition to intraframe redundancies. As an example, MPEG-2 and similar compression techniques reduce interframe redundancy by determining the differences between two or more images based on motion information. This combination of information from different images due to interframe compression, has the effect of averaging the noise introduced by watermarks. Such operations can be considered attacks (whether malicious or non-malicious) on any watermarks.